In a complex, large industrial plant such as a pressurized water nuclear reactor, unwanted incidental phenomena may occur, such as fracturing, friction between neighboring parts or impact between a structural element of the reactor and a mechanical member such as a bolt, a washer or a valve element which becomes detached from the structure of the reactor and moves at high speed within it. Such a mechanical member detached from the structure of the reactor is carried along by the cooling water of the reactor which circulates at very high speed.
In practice, in plants such as pressurized water nuclear reactors, because of the stresses experienced by parts in service, these being stresses of thermal or mechanical origin, for example, more or less complete local fractures may occur which can develop to the point of a part becoming detached from the structure and being carried along by the flow to become a migrant body in the plant. The effect of stresses is also increased by corrosion.
Also, before the part becomes completely detached from the structure, or where the connection between the part and the structure is being completely destroyed, friction or impact may occur between the part becoming detached and the part of the plant located near it.
In all instances, it is essential to detect the fracture, friction or impact phenomena as rapidly as possible and to monitor their development before they cause significant damage in the plant.
The use has long been proposed of acoustic sensing devices positioned on the structure of the plant or microphones positioned in the atmosphere near the structure in which these destructive phenomena may occur, to sense or record the acoustic waves accompanying the incidental phenomena requiring very rapid detection.
For example, acoustic sensing devices for supplying a signal in response to excitation by acoustic waves have been installed near the regions in which the probability of impact is large or near parts with a considerable probability of fracture.
In a complex industrial plant which is in operation, however, acoustic waves may be produced at various locations in the plant, because, for example, machine members such as pumps or fans are in practically continuous operation.
In the case of nuclear reactors, noise of very varied origin, such as noise accompanying closure of the flap of a valve, the movement of control rods or other movable members can create acoustic signals of the impulse type which are observed at the sensing device terminals and which can be taken for signals resulting from an impact or friction between two parts or from cracks propagated at the moment when a part fractures.
It is very difficult to discriminate effectively between signals due to incidental phenomena and signals due to other causes whose origin can be located in a remote part of the plant.
In particular, currently known apparatuses do not allow a weak acoustic excitation whose source is nearby to be distinguished from a greater excitation created a long way off.
For example, in the case of nuclear reactors, known apparatuses do not allow distinguishing of acoustic waves due to an impact on the bottom of a steam generator and closure of a valve flap a long way from the region in which the sensing device is located. The use of recordings from sensing devices by an electronic monitoring apparatus therefore makes it necessary to take account of the behavior of signals and to analyze them by using an oscilloscope.
In addition, electrical interference may be perceived by the monitoring chain as an acoustic pulse which causes false alarms.
Apparatuses have also been proposed which can be used in the field of non-destructive testing of materials which allow a source of acoustic emissions to be exactly located, for example a fault in the structure of material which emits acoustic waves under the effect of stress.
For this technique, an array of sensing devices is disposed in contact with the structure or material being tested, the time intervals separating the arrival of acoustic waves at the various sensing devices of the array are measured, and these measurements are processed to locate the source of acoustic emissions.
However, such a unit for measuring and processing acoustic signals is very complex, and in the case of an industrial plant, a processing unit of very great capacity, and hence very expensive, would have to be used to obtain accurate location of each source emitting an acoustic signal picked up by the set of sensing devices.
Furthermore, distinguishing various acoustic signals by locating their emission source would also pose problems which would be difficult to solve technically, if very rapid processing and triggering of an alarm is required before the recorded phenomenon has developed to any great extent.